The present invention relates to hydrostatic transmissions (HSTs), and more particularly, to such transmissions which are used in connection with the propulsion of vehicles such as lawn and garden tractors.
A typical hydrostatic transmission includes a pair of fluid displacement mechanisms, one of which functions as a pump and the other of which functions as a motor. Conventionally, the pump comprises a variable displacement hydrostatic unit, while the motor comprises a fixed displacement hydrostatic unit. Therefore, there are typically various controls associated with the pump, such as some form of displacement control, and perhaps other controls as well. One example of another control which would be present on a typical HST would be a "dump" control, which generally is well known in the art, and is used to interconnect the two sides of the hydrostatic loop (or connect both sides to the system reservoir), thus making it possible to push or tow the vehicle without having to overcome the dynamic braking effect of the hydrostatic transmission in its normal operating mode.
Although the present invention may be used advantageously with hydrostatic transmissions including various types of fluid displacement mechanisms, it is especially advantageous when the fluid displacement mechanisms comprise radial piston devices, and the invention will be described in connection therewith. As used herein, the term "radial piston" will be understood to include devices in which the piston is either a true cylindrical piston which reciprocates in a cylinder, or a ball member which reciprocates in a cylinder.
One of the problems associated with providing hydrostatic transmissions to vehicle original equipment manufacturers (OEMs) is that each OEM normally has its own preferred linkage arrangement already designed into its vehicle. For example, one OEM may utilize a pump displacement control in which depressing a foot pedal pushes or pulls a linkage member "forward" for the purpose of increasing pump displacement in a forward direction. Another vehicle OEM may have a pump displacement control in which depressing a foot pedal shifts a linkage member "rearward" to increase pump displacement in the forward direction.
Typically, the manufacturers of HSTs have provided the pumps of their transmissions with pump displacement controls which move one direction to increase pump displacement moving forward and move in the opposite direction to increase pump displacement moving in reverse. If the potential OEM customer already has its control linkage set up for movement corresponding to what is already available on the HST, there are no linkage changes required. If, on the other hand, the HST manufacturer has its pump displacement control set up to move in a direction opposite to that of the linkage being used by the vehicle OEM, it is necessary for the HST manufacturer to request the vehicle OEM to redesign and reverse the vehicle linkage to accommodate the HST linkage.
Accordingly, it is an object of the present invention to provide an improved hydrostatic transmission which can be used on a vehicle having either forward-moving linkage or rearward-moving linkage, without any substantial redesign or revision of the hydrostatic transmissions, and which eliminates the need for the vehicle OEM to redesign or reverse its linkage.
The above and other objects of the invention are accomplished by the provision of an improved hydrostatic transmission comprising a housing assembly, a variable displacement radial piston pump, and a fluid pressure-operated motor disposed in the housing assembly. The radial piston pump includes displacement varying means operable to vary the displacement of the pump, including reversible control means operable to move the displacement varying means in first and second opposite directions, in response to movement of the control means in first and second opposite directions from a neutral position. The motor defines a fluid inlet and a fluid outlet, and the housing assembly defines high- and low-pressure passages in fluid communication with the fluid inlet and fluid outlet, respectively. The pump comprises a rotor assembly rotatably mounted on a journal member disposed within a journal bore defined by the housing assembly, the journal bore providing open fluid communication with both of the high- and low-pressure passages. The journal member defines high- and low-pressure pump slots in fluid communication with contracting and expanding chambers, respectively, of the pump, the journal member further defining high- and low-pressure housing slots in fluid communication with the high- and low-pressure passages, respectively, and the journal member defining a high-pressure axial bore interconnecting the high-pressure pump and housing slots, and a low-pressure axial bore interconnecting the low-pressure pump and housing slots.
The improved hydrostatic transmission is characterized by operating in a forward direction in response to movement of the control means in the first direction from the neutral position in the presence of a first journal member disposed in the journal bore wherein the axial bores are oriented generally perpendicular to the pump slots, and the housing slots are oriented generally perpendicular to the axial bores. The hydrostatic transmission is further characterized by operating in the forward direction in response to movement of the control means in the second direction from the neutral position in the presence of a second journal member disposed in the journal bore wherein the axial bores are oriented at an acute angle relative to the pump slots, and the housing slots are oriented at an acute angle relative to the axial bores.
In the typical prior art dump valve (unloading valve) arrangement, such as is shown in U.S. Pat. No. Re 28,953, the hydrostatic loop is "unloaded" by simultaneously unseating both of the anti-cavitation ball check valves. In the prior art, the mechanism which unseats the ball check valves has typically comprised structure which added substantially to the cost and complexity of manufacture and assembly of the pump. Furthermore, the typical prior art dump valve mechanism added substantially to the size of the pump and quite often involved members both internal and external to the pump housing, such that a fairly complex arrangement of seals was required.
Accordingly, it is an object of the present invention to provide an improved hydrostatic transmission including a dump valve (unloading valve) arrangement which overcomes the above-described disadvantages of the prior art.
The above and other objects of the invention are accomplished by the provision of a hydrostatic transmission comprising a housing assembly, a fluid pressure-operated pump, and a fluid pressure-operated motor disposed in the housing assembly. The pump defines a fluid inlet and a fluid outlet and the motor defines a fluid inlet and a fluid outlet. The housing assembly defines a case chamber, a first passage providing fluid communication from the pump fluid outlet to the motor fluid inlet, and a second passage providing fluid communication from the motor fluid outlet to the pump fluid inlet. A first check valve means is operable to permit fluid communication from the case chamber to the first passage in the absence of fluid pressure in the first passage, and a second check valve means is operable to permit fluid communication from the case chamber to the second passage in the absence of fluid pressure in the second passage. A dump mechanism is operably associated with the first and second check valve means to open both of said check valve means simultaneously in response to an input.
The improved hydrostatic transmission is characterized by the housing assembly defining an insertion chamber adjacent said case chamber and in open communication therewith, and further defining retention means, the insertion chamber being disposed between the case chamber and the retention means. The dump mechanism comprises a unitary dump member including a dump portion including first and second projecting portions disposed for unseating engagement with the first and second check valve means, respectively, in response to movement of the dump portion toward the check valve means. The unitary dump member further includes a pair of resiliently deflectable mounting portions, each including a retention member, the mounting portions being deflectable during insertion thereof through the insertion chamber until the retention members engage the retention means, the unitary dump member thereafter being pivotable about the retention members. The dump mechanism includes means biasing the dump portion away from the check valve means, and the dump mechanism includes means operable in response to the input to cause the dump member to pivot about the retention members and move the projecting portions toward the check valve means.